Heat ablative compositions utilizing a binder and a filler consisting of aluminum sulfate hydrate are known. See U.S. Pat. No. 4,462,831 and U.K. Patent Application No. 2,079,294.
Heat ablative compositions are the most effective type of heat-isolative materials for intense heat ranging from 700.degree. to 1500.degree. C., for example, heat due to fire. Whereas conventional heat isolative materials protect by insulating, heat ablative compositions protect by absorbing heat as a result of physical and chemical endothermic degradation of the binder and filler. The endothermic degradation absorbs practically all the heat applied to the composition up to the moment when the composition is exhausted. For example in U.S. Pat. No. 4,462,831, endothermic degradation occurs when the aluminum sulfate hydrate is dehydrated.
Heat ablative compositions utilize a number of processes to protect against heat. The following is an illustrative list of such processes, and is not meant to be all-inclusive:
1. Chemical destruction of the binder; PA0 2. The absorbtion of heat caused by the liberation of water molecules from the hydrated inorganic salt, i.e., heat of dehydration; PA0 3. The absorbtion of heat caused by the evaporation of the liberated water molecules, i.e., heat of evaporation; PA0 4. Foam formation in the ablative composition; PA0 5. The flow of yielding gases out of the ablative composition, which carries heat away from the composition; and PA0 6. Color change of the ablative composition to white, which reflects heat.
Absorbtion of heat of dehydration and absorbtion of heat of evaporation by hydrated inorganic salts involve the loss of water of hydration. The temperature at which the heat absorbtion occurs is typically about 100.degree. C., the boiling point of water. Thus, heat ablative compositions containing hydrated inorganic salts absorb heat once the composition reaches a temperature of about 100.degree. C. The temperature of the composition remains at approximately 100.degree. C. until all water is driven off.
The efficiency of ablative compositions is characterized by the elapsed time between the initiation of heating of the composition and the point at which the composition reaches a given maximum temperature. This efficiency rating is known in art as the "Fire Protective Rating" (F.P.R.), and is measured in, for instance, minutes. The American Society of Testing Materials (ASTM) has standardized the testing procedure for determining F.P.R. in ASTM E-119 and ASTM E-814.
One type of filler used in ablative composition is an inorganic hydrated salt, aluminum sulfate hydrate, Al.sub.2 (SO.sub.4).sub.3 -14-18H.sub.2 O. See U.S. Pat. No. 4,462,831. A filler consisting solely of aluminum sulfate hydrate, however, has certain deficiencies. Aluminum sulfate hydrate is highly acidic, containing about 48.5% sulfate ions. Its efficacy is greatest when the water of crystallization is maximized. Moreover, aluminum sulfate hydrate has a tendency to dehydrate during storage.
The acidic character of aluminum sulfate hydrate has numerous consequences on the performance of the heat ablative composition. The acidity may cause the destruction of the binder, which results in decreased F.P.R.'s. Aluminum sulfate hydrate's acidity may also result in cracking, brittleness and total loss of impact resistance of the binder, and loss of tensile strength, compressive strength or bending strength in the finished ablative product. The excess acidity may result in erosion or destruction by oxidation of the substructure to which the ablative composition is applied.
Aluminum sulfate hydrate is most effective when the molar ratio of water to aluminum sulfate is 18 to 1. As the ratio drops from 18:1 to 14:1, the F.P.R. decreases by as much as one fourth or more. Moreover, aluminum sulfate hydrate having a water to sulfate ratio of 18:1 is not commercially available. As a general rule commercially available aluminum sulfate hydrate has a water to sulfate ratio between 14 and 14.5 to 1.
The term "aluminum sulfate hydrate", as used herein, refers to compounds having the general formula Al.sub.2 (SO.sub.4).sub.3 .multidot.nH.sub.2 O wherein n is from 14 through 18, inclusive.
Aluminum ammonium sulfate hydrate (AlNH.sub.4 (SO.sub.4).sub.2 -12H.sub.2 O) is also useful as a filler in ablative compositions. Its most significant disadvantage is its low F.P.R. However, it has a somewhat lower acidic character than aluminum sulfate hydrate, by virtue of a lower sulfate ion content (42.4% v. 48.5%). Aluminum ammonium sulfate hydrate has a greater stability than aluminum sulfate hydrate, and a more constant water of crystallization. Moreover, it is readily available, generally at inexpensive cost.